Several experimental efforts related to the concrete improvement are focused to increase its flexural strength to complement the high compressive strength, which is usually developed by materials of this nature. The flexural strength or modulus of rupture of the concrete is important in civil engineering applications such as infrastructure projects, pavements and buildings. This work proposes an alternative to optimize concrete flexural strength through the functionalization of the 9 Angstrom (A) Tobermorite using Carbon Nanotubes (CNT). A complete ab-initio, 3D Atomistic Model of the 9 A Tobermorite is presented as the basis of the silicate cementitious hydrated products. In order to validate the model, some mechanical properties were computed using a Density Functional Theory (DFT) based program. Afterwards, a functionalization based on CNTs with different diameters was carried out to improve the flexural strength of the concrete.